An efficient floating adsorption-photocatalyst to decarboxylate D-Glu and D-MeAsp of Microcystin-LR via holes direct oxidation

Abstract

Fates of toxic foundational groups of Microcystin-LR in aqueous have garnered widespread attention because of the difficulty of removal during photocatalytic process. Here, an efficient N, P co-doped TiO2/expanded graphite (NPTEG) floating material (NPTEG) modified by poly dimethyl diallyl ammonium chloride (PDDA) and graphitic carbon nitride (g-C3N4) (x PDDA-g-C3N4-NPTEG) was assembled and used to destroy the toxic foundational groups of MC-LR in water. g-C3N4 can match well with TiO2 to form heterojunction where holes can gather in the valence band of g-C3N4. PDDA is used to passivate the g-C3N4, and performs as holes transport layer during the photocatalytic process to promote the separation of electron-hole pairs. Under the direct interaction of photo-generated holes and free carboxyl groups of MC-LR adsorbed on the surface, the decarboxylation process in the free acid groups on D-glutamic acid (Glu) and methyl-D-aspartic acid (D-MeAsp) of MC-LR occurred which was authenticated by UPLC-ESI-MS/MS. Meanwhile, the conjugated double bond was also removed by hydroxylation, so as to achieve the goal of non-toxic site residues in the degradation process. Furthermore, the degradation byproducts toxicity and photocatalysts stability were explored. The results confirm that x PDDA-g-C3N4-NPTEG exhibits excellent attacking ability of carboxylic acid groups of MC-LR and high recycle ability, which has great promise as a means of effectively treating drinking water.